In the electronics manufacturing industry, printed wiring boards, also known as printed circuit boards, are frequently used for mounting large numbers of devices such as hybrid circuits, integrated circuits, individual components and the like. A printed wiring board normally contains a pattern of conductive traces on the surfaces of the board; and the board acts as a dielectric material for electrically coupling the various devices in a desired configuration. Two or more printed wiring boards can be interconnected through connecting pads, connectors and a backplane. A printed wiring board can comprise either a single dielectric sheet or a plurality of dielectric sheets laminated together into a more or less rigid laminated board. The sheets carry the conductive traces or paths which interconnect the component pads affixed to the board. Some of the conductive paths connect with connecting pads which are located on the board at or near an edge of the board for purposes of making connections to circuitry located external to the board. Frequently only one of the 4 edges is available for such connections. It is desirable to establish such connection at arbitrary locations throughout the interior of the board.
In data processing systems, the need has arisen to transmit to or receive from arbitrary locations on a printed wiring board high speed data streams having bit rates which extend into the microwave region, for example, a 2.488 GHz clock signal and data connections at the electro-optic interfaces of light wave systems. In these instances the conductive traces are designed to perform as controlled impedance transmission lines. A controlled impedance transmission line retains the desired characteristic impedance (for example, 50 ohms) at the interconnection to frequencies extending into the microwave region. Examples of controlled impedance transmission lines are a strip transmission line, a microstrip transmission line and a coplanar waveguide transmission line.
In one type of assembly, the printed wiring board can include, as the conductive path, a microstrip transmission line affixed to one side of a dielectric sheet and a relatively wide flat conductor affixed to the opposite side of the dielectric sheet. A second dielectric sheet is positioned against over the side of the first dielectric sheet having the relatively wide flat conductor and a connector for a coaxial transmission line is coupled to the exposed side of the second dielectric sheet. It is to be noted that the exposed side of the second dielectric sheet is not required to support any conductive paths. The coaxial connector is coupled to the conductive paths which can be a trace or strip transmission line located on the far side of the board assemblage by means of an opening in the board through which conductive wires can pass. One wire extends through the opening in the dielectric sheets from the center lead of the coaxial connector to a conductive trace on the far side of the first dielectric sheet. A second connection is made from the body of the coaxial connector to a conductive pad also located on the far side of the first dielectric sheet, for coupling the body of the connector to the relatively wide flat conductor which defines the ground plane located between the two dielectric sheets.
The conductors which pass through the dielectric sheets to connect the coaxial connector to the strip transmission line and ground plane cause an abrupt change in the physical characteristics of the line which, in turn, cause an abrupt change in the characteristic impedance of the line. The line, therefore, losses its controlled impedance performance. This change introduces objectionable electrical performance and losses to the signal being propagated between the strip transmission line and the coaxial transmission line. An improved coaxial transmission line to strip transmission line coupler is required to reduce this deleterious condition.